1. Technical Field of the Invention
The present invention relates to an etching method using plasma for the manufacture of electronic devices such as semi-conductors or micro-machines, and to a cleaning method for the inner wall of a vacuum chamber constituting a plasma processing apparatus, and further to a plasma processing apparatus and a matching circuit for matching high-frequency impedance of load.
2. Description of Prior Art
One example of a conventional high-frequency induction type plasma processing apparatus is shown in FIG. 7 and FIG. 8. In FIG. 7, a gas feeding apparatus 2 introduces a prescribed gas into a vacuum chamber 1 while the vacuum chamber 1 is evacuated by a pump 3 as an evacuation apparatus so as to maintain a predetermined pressure within the vacuum chamber 1. High-frequency electric current is supplied to a dome-like spiral coil 6 disposed on an induction window 5 from a high-frequency power source 4 thereby to generate plasma in the vacuum chamber 1, with which plasma processing can be performed to a substrate 8 placed on a substrate electrode 7, such as etching, deposition, and surface alteration. At the same time, high-frequency electric power is supplied to the substrate electrode 7 through a matching circuit 9 from a high-frequency power source 10, so as to control ion energy beamed on the substrate 8. The plasma processing apparatus of such type is described in detail in Japanese Laid-open Patent Applications 8-83696 and 9-82692.
FIG. 8 shows a detailed diagram of the substrate electrode 7 and matching circuit 9. The substrate electrode 7 comprises a pedestal 12, an insulating member 13, a first chuck electrode 14, and a second chuck electrode 15. The pedestal 12 is connected to a high-frequency output terminal 16 of the matching circuit 9. The substrate 8 is held on the substrate electrode 7 by electrostatic chuck by applying high voltage DC voltage (hundreds to thousands V) of different polarities to the first and second chuck electrodes 14, 15 from a DC power source 17. The matching circuit 9 comprises two variable capacitors 18, 19, a coil 20, a high-frequency input terminal 21, and the high-frequency output terminal 16. The high-frequency input terminal 21 is connected to the high-frequency power source 10. Electrostatic chuck helps enhance the heat-conductivity between the substrate 8 and substrate electrode 7, thereby preventing an excessive raise in temperature of the substrate 8 caused by plasma. Temperature control of the substrate 8 can be more accurately conducted by reserving hundreds Pa of helium gas between the chucked substrate 8 and the substrate electrode 7, thereby further increasing the heat conductive effects.
FIG. 9 shows a mechanical clamp type apparatus, in which the substrate 8 is held on the substrate electrode without using electrostatic chuck. As shown in FIG. 9, the substrate 8 is pressed down on the substrate electrode 7 with a clamp ring 29 by means of a tension force of a spring 28. The surface of substrate electrode 7 is normally covered with an insulating material. Similarly to the electrostatic chuck type apparatus, it is possible to conduct temperature control of the substrate 8 by enhancing the heat conductive effects between the substrate 8 and substrate electrode 7, by reserving helium gas therebetween.
The prior art methods shown in FIG. 7 through FIG. 9 had a problem that the self-biasing potential generated in the substrate cannot be monitored. Specific explanation of this problem is given below.
When high-frequency electric power is supplied to a solid material in contact with plasma, negative DC potentials are generated in the solid material due to the matching effects which are caused by differences in transferring amount of electrons and ions. This DC potential is called self-biasing potential. The self-biasing potential reflects the voltage-current characteristics of plasma and thus reacts sensitively to the change in composition and density of particles in plasma. During the etching process, a substance generated by etching reaction is present in a gaseous state within the vacuum chamber. Once the etching reaction has been completed and the ground surface is exposed, the density of the substance generated by the etching reaction decreases, and the voltage-current characteristics of plasma changes, which affects and causes the self-biasing potential to change even though the amount of high-frequency electric power supplied to the substrate electrode 7 is unchanged. It is thus possible to detect a final point of etching by monitoring the self-biasing potential. Monitoring of self-biasing potential is also effective as a measure to detect a final point of cleaning in the case where plasma is used to clean the inner wall of vacuum chamber 1 contaminated as a result of etching or CVD processing. It is a normal practice to perform such cleaning using a dummy substrate placed on the substrate electrode, in order to prevent wear of the substrate electrode 7.
Despite these benefits of obtaining useful information by monitoring the self-biasing potential, it is not possible in the conventional apparatuses shown in FIG. 7 through FIG. 9 to monitor the self-biasing potential generated in the substrate. In the electrostatic chuck type apparatus, the DC potential of the substrate 8 cannot possibly measured since the substrate 8 is placed on the insulating member 13 constituting the substrate electrode 7. In the mechanical clamp type apparatus, too, the surface of the substrate electrode 7 is covered with an insulating material and thus the DC potential of the substrate 8 is not measurable. It would be possible to measure the DC potential of the substrate 8 if the insulating material on the surface of the substrate electrode 7 were removed thus allowing conduction between the substrate 8 and the electrode 7, but such construction would be problematic for the following reason. The substrate 8 and the substrate electrode 7 are in contact with each other in an extremely small area since the surfaces thereof are not completely even, and if an electric current is concentrated on the contact areas, it will destroy the device on the substrate such as a transistor or the like (so-called charge-up damage).
Japanese Laid-open Patent Application 3-74843 discloses a method of monitoring the self-biasing potential by directly contacting a metal terminal to the backside of the substrate 8, but with this method even, the charge-up damage cannot be fully prevented.